IETF Next Steps in Signaling C. Shen
Internet-Draft H. Schulzrinne
Intended status: Standards Track Columbia U.
Expires: April 24, 2007 S. Lee
J. Bang
Samsung AIT
October 21, 2006
NSIS Operation Over IP Tunnelsdraft-ietf-nsis-tunnel-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This draft presents an NSIS operation over IP tunnel scheme using QoS
NSLP as the NSIS signaling application. Both sender-initiated and
receiver-initiated NSIS signaling modes are discussed. The scheme
creates individual or aggregate tunnel sessions for end-to-end
sessions traversing the tunnel. Packets belonging to qualified end-
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Internet-Draft NSIS Operation over IP Tunnels October 20061. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [1].
2. Introduction
When IP tunnel mechanism is used to transfer signaling messages,
e.g., NSIS messages, the signaling messages usually become hidden
inside the tunnel and are not known to the tunnel intermediate nodes.
In other words, the IP tunnel behaves as a logical link that does not
support signaling in the end-to-end path. If true end-to-end
signaling support is desired, there needs to be a scheme to enable
signaling at the tunnel segment of the end-to-end signaling path.
This draft describes such a scheme for NSIS operation over IP
tunnels. We assume QoS NSLP as the NSIS signaling application.
2.1. IP Tunneling Mechanisms and Tunnel Signaling Capability
There are a number of common IP tunneling mechanisms, such as Generic
Routing Encapsulation (GRE) [4][15], Generic Routing Encapsulation
over IPv4 Networks (GREIP4) [5] , IP Encapsulation within IP
(IP4INIP4) [7], Minimal Encapsulation within IP (MINENC) [8], Generic
Packet Tunneling in IPv6 Specification (IP6GEN) [11], IPv6 over IPv4
tunneling (IP6INIP4) [9], IPSEC tunneling mode [19][10]. These
mechanisms can be differentiated according to the format of the
tunnel encapsulation header. IP4INIP4, IP6INIP4 and IP6GENIP4 can be
seen as normal IP in IP tunnel encapsulation because their tunnel
encapsulation headers are in the form of a standard IP header. All
GRE-related IP tunneling (GRE,GREIP4), MINENC and IPSEC tunneling
mode can be seen as modified IP in IP tunnel encapsulation because
the tunnel encapsulation header contains additional information
fields besides a standard IP header. The additional information
fields are the GRE header for GRE and GREIP4, the minimum
encapsulation header for MINENC and the Encapsulation Security
Payload (ESP) header for IPSEC tunneling mode.
By default any end-to-end signaling messages arriving at the tunnel
endpoint will be encapsulated the same way as data packets. Tunnel
intermediate nodes do not identify them as signaling messages. A
signaling-aware IP tunnel can participate in a signaling network in
various ways. Prior work on RSVP operation over IP tunnles (RSVP-
TUNNEL) [16] identifies two types of QoS-aware tunnels: a tunnel that
can promise some overall level of resources but cannot allocate
resources specifically to individual data flows, or a tunnel that can
make reservations for individual end-to-end data flows. This
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classification leads to two types of tunnel signaling sessions:
individual tunnel signaling sessions that are created and torn down
dynamically as end-to-end session come and go, and aggregate tunnel
sessions that can either be fixed, or dynamically adjusted as the
actually used session resources increase or decrease. Aggregate
tunnel sessions are usually pre-configured but can also be
dynamically created. A tunnel MAY contain only individual tunnel
sessions or aggregate tunnel sessions or both.
2.2. NSIS Tunnel Operation Overview
This NSIP operation over IP tunnel scheme is designed to work with
most, if not all, existing IP in IP tunneling mechanisms. The scheme
requires the tunnel endpoints to support specific tunnel related
functionalities. Such tunnel endpoints are called NSIS-tunnel
capable endpoints. Tunnel intermediate nodes do not need to have
special knowledge about this scheme. When tunnel endpoints are NSIS-
tunnel capable, this scheme enables the proper signaling initiation
and adjustment inside the tunnel to match the requests of the
corresponding end-to-end session. In cases when tunnel session
signaling status is uncertain or not successful, the end-to-end
session will be notified about the existence of possible NSIS-unaware
links in the end-to-end path.
The overall design of this NSIS operation over IP tunnel scheme is
conceptually similar to RSVP-TUNNEL [16]. However, the details of
the scheme address all the important differences of NSIS from RSVP.
For example,
o NSIS is based on a two-layer architecture, namely a signaling
transport layer and a signaling application layer. It is designed
as a generic framework to accommodate various signaling
application needs. The basic RSVP protocol does not have a layer
split and is only for QoS signaling.
o NSIS QoS NSLP allows both sender-initiated and receiver-initiated
reservations; RSVP only supports receiver-initiated reservations.
o NSIS deals only with unicast; RSVP also supports multicast.
o NSIS integrates new features, such as the Session ID, to
facilitate operation in specific environments (e.g. mobility and
multi-homing).
From a high level point of view, there are two main issues in a
signaling operation over IP tunnel scheme. First, how packet
classification is performed inside the tunnel. Second, how signaling
is carried out inside the tunnel.
Packets belonging to qualified data flows need to be recognized by
tunnel intermediate nodes to receive special treatment. Packet
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classification is traditionally based on flow ID. After a typical
IP-in-IP tunnel encapsulation, packets from different flows appear as
having the same flow ID which usually consists of the Tunnel Entry
(Tentry) address and Tunnel Exit (Texit) address. Therefore, the
flow ID for a signaled flow needs to contain further demultiplexing
information to make it distinguishable from non-signaled flows, and
also from other different signaled flows.
The special flow ID for signaled flows inside the tunnel needs to be
carried in tunnel signaling messages, along with tunnel adjusted QoS
parameters, to set up or modify the state information in tunnel
intermediate nodes. This process creates separate tunnel signaling
sessions between the tunnel endpoints. In most cases, it is
necessary to maintain the state association between an end-to-end
session and its corresponding tunnel session so that any change to
one session MAY be reflected in the other.
In the next section, we will illustrate details on packet
classification over the tunnel, signaling over the tunnel as well as
association of end-to-end and tunnel signaling.
3. Protocol Design Decisions3.1. Flow Packet Classification over the Tunnel
A flow can be an individual flow or an aggregate flow. Flow ID
formats that MAY be used to identify packets in individual tunnel
flows are listed below.
o Selected fields from the base IP header portion of the tunnel
encapsulation header. For example, the IP source and destination
address fields, which contain the IP addresses of Tentry and
Texit, together with another field for tunnel-wide demultiplexing.
This could be the IPv6 flow label field [6], or the Traffic Class,
also known as DiffServ Code Point (DSCP) field. Note that the
DSCP field can also be used to represent an aggregate DiffServ
flow. As long as individual flow classification is processed
before aggregate flow classification, or a longest match kind of
packet classifier is used, this individual tunnel flow
demultiplexing with DSCP field can work. In the rare cases where
these conditions cannot be satisfied, it is still possible to
choose different range of DSCP values so that the values used for
individual tunnel flow demultiplexing do not collide with those
used for DiffServ aggregate flows. Compared to the IPv6 flow
label approach, using DSCP field as part of the tunnel flow ID can
be applied to both IPv4 and IPv6 and is probably easier to deploy.
The drawback is that the small number of bits in the DSCP field
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limits the total number of individual flows that can be
distinguished in the tunnel. Overall, this group of flow ID
formats enable efficient packet classification over the tunnel
without introducing additional processing requirements on the
existing infrastructure. They are also easy to deploy.
o Selected fields from the base IP header portion of the tunnel
encapsulation header, combined with fields from the addtional
infromation in the tunnel encapsulation header. This applies to
modified IP-in-IP encapsulation as we mentioned in Section 2.1.
An example of the additional information field is the Security
Prameter Index (SPI) field for IPSEC tunnels. Comparing with the
flow ID formats in the first group, these flow ID formats might
pose more requirements at the NSIS protocol side if the addition
information field is unique to the specific tunnel mechanism and
not already recognized in basic NSIS specification.
o UDP header insertion. Inserting an extra UDP header between the
tunnel encapsulation IP header and the tunnel payload provides
additional demultiplexing information for a tunnelled flow. The
drawback of this flow ID format, as compared to the above two
format groups, is the additional UDP header overhead both for
bandwidth and processing. Moreover, this approach modifies the
basic tunneling mechanism at the Tentry, so Texit MUST also be
aware of the special UDP insertion in order to correctly
decapsulate and forward original packets further along the path.
The above three groups of flow ID formats MAY also be used for
aggregate tunnel flows. For example, a common aggregate flow ID
contains the addresses of tunnel endpoints and a DSCP value. There
are other options for aggregate flows. For example, When additional
interfaces at tunnel endpoints are available, the IP address of an
additional interface at Tentry plus the IP address of the Texit, MAY
constitute an aggregate flow ID.
The decision of using a specific flow ID format is left to a policy
mechanism outside the scope of this document. Tunnel signaling is
performed based on the chosen flow ID. As long as the flow ID format
is supported, Tentry SHOULD encapsulate all incoming packets for the
specific data flows according to the chosen flow ID format. Texit
SHOULD be able to decapsulate the packets if any special tunnel flow
encapsulation is performed at the Tentry.
3.2. Tunnel Signaling and its Association with End-to-end Signaling
Tunnel signaling messages contain tunnel specific parameters such as
tunnel Message Routing Information (MRI) and tunnel adjusted QoS
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parameters. But in general, the formats of tunnel signaling messages
are the same as end-to-end signaling messages. Tunnel signaling is
carried out according to the same signaling rules as for end-to-end
signaling. The main challenge is, therefore, the interaction between
tunnel signaling and end-to-end signaling. The interaction is
achieved by special functionalities supported in the NSIS-tunnel
aware tunnel endpoints. These special functionalities include
assigning tunnel flow IDs, creating tunnel session association,
notifying the other endpoint about tunnel association, adjusting one
session based on change of the other session, encapsulating
(decapsulating) packets according to the chosen tunnel flow ID at
Tentry (Texit), and etc. In most cases, we expect to have bi-
directional tunnels, where both tunnel endpoints are NSIS-tunnel
aware.
When both Tentry and Texit are NSIS-tunnel aware, the endpoint that
creates the tunnel session MAY need to notify the other endpoint of
the association between the end-to-end and tunnel session. This is
achieved by using the QoS NSLP BOUND_SESSION_ID object with a binding
code indicating tunnel handling as the reason for binding. In the
rest of this document, we refer to a BOUND_SESSION_ID object with its
tunnel binding_code set as a tunnel BOUND_SESSION_ID object or a
tunnel binding object. The tunnel binding object is carried in the
end-to-end signaling messages and contains the session ID of the
corresponding tunnel session. NSIS-tunnel aware endpoints that
receive this tunnel BOUND_SESSION_ID object SHOULD perform tunnel
related procedures and then remove it from any end-to-end signaling
messages sent out of the tunnel.
4. Protocol Operation with Dynamically Created Tunnel Sessions
The operation details for NSIS signaling over IP tunnels are more
complicated if the tunnel session needs to be dynamically created,
comparing to the case when tunnel sessions are pre-configured. We
discuss these two cases in this and the subsequent section,
respectively. If a tunnel contains both dynamic and pre-configured
tunnel sessions, it can be handled by the combination of the
corresponding mechanism for each type of tunnel sessions. The choice
of mapping an end-to-end session to a specific type of tunnel session
is up to policy control.
4.1. Operation Scenarios
To dynamically create a mapping tunnel session upon receiving an end-
to-end session, we identify four scenarios based on the sender-
initiated and receiver-initiated reservation modes of NSIS QoS NSLP:
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o End-to-end session is sender-initiated; tunnel session is sender-
initiated.
o End-to-end session is receiver-initiated; tunnel session is
receiver-initiated.
o End-to-end session is sender-initiated; tunnel session is
receiver-initiated.
o End-to-end session is receiver-initiated; tunnel session is
sender-initiated.
In the following we describe a typical NSIS end-to-end and tunnel
signaling interaction process during the tunnel setup phase in each
of these four scenarios. The end-to-end QoS flow is assumed to be
one that qualifies an individual dynamic tunnel session, whose tunnel
reservation MUST be confirmed before the end-to-end reservation can
proceed further outside the tunnel.
It SHOULD be noted that different flow QoS requirements and policy
assumptions MAY cause the timing sequence of the messaging flow to be
slightly different. This will be discussed in Section 4.2.
Once the tunnel session has been created and associated with the end-
to-end session, any subsequent changes (modification or termination)
to either session MAY be communicated to the other one by the binding
endpoint so the state of the two binding sessions can keep
consistent. The exception is when the tunnel session is an aggregate
session. In that case, after setup, the adjustment of the tunnel
session SHOULD follow the rules for pre-configured aggregate tunnel
adjustment in Section 5.
4.1.1. Sender-initiated Reservation for both End-to-end and Tunnel Signaling
Sender Tentry Tnode Texit Receiver
| | | | |
| RESERVE | | | |
+--------->| | | |
| | RESERVE' | | |
| +=========>| | |
| | | RESERVE' | |
| | +=========>| |
| | RESERVE | |
| +-------------------->| |
| | | RESPONSE'| RESERVE |
| | |<=========+--------->|
| | RESPONSE'| | |
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| |<=========+ | |
| | | | RESPONSE |
| | | |<---------+
| | RESPONSE | |
| |<--------------------+ |
| RESPONSE | | | |
|<---------+ | | |
| | | | |
| | | | |
Figure 1: Sender-initiated Reservation for both End-to-end and Tunnel
Signaling
This scenario assumes both end-to-end and tunnel sessions are sender-
initiated. Figure 1 shows the messaging flow of NSIS operation over
IP tunnels in this case. Tunnel signaling messages are distinguished
from end-to-end messages by a "'" after the message name. Tnode
denotes an intermediate tunnel node that participates in tunnel
signaling. The sender first sends an end-to-end RESERVE message
which arrives at Tentry. If Tentry supports tunnel signaling and
determines that an individual tunnel session needs to be established
for the end-to-end session, it chooses the tunnel flow ID, creates
the tunnel session and associates the end-to-end session with the
tunnel session. It then sends a tunnel RESERVE' message matching the
requests of the end-to-end session toward the Texit to reserve tunnel
resources. Tentry also appends to the original RESERVE message a
tunnel BOUND_SESSION_ID object containing the session ID of the
tunnel session and sends it toward Texit using normal tunnel
encapsulation.
The tunnel RESERVE' message is processed hop by hop inside the tunnel
for the flow identified by the chosen tunnel flow ID. When Texit
receives the tunnel RESERVE' message, a reservation state for the
tunnel session will be created. Texit MAY also send a tunnel
RESPONSE' message to Tentry. On the other hand, the end-to-end
RESERVE message passes through the tunnel intermediate nodes just
like any other tunneled packets. When Texit receives the end-to-end
RESERVE message, it notices the binding of a tunnel session and
checks the state for the tunnel session. When the tunnel session
state is available, it updates the end-to-end reservation state using
the tunnel session state, removes the tunnel BOUND_SESSION_ID object
and forwards the end-to-end RESERVE message further along the path
towards the receiver. When the end-to-end reservation finishes, an
end-to-end RESPONSE MAY be sent back from the receiver to the sender.
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Tentry also appends to the original QUERY message with a tunnel
BOUND_SESSION_ID object containing the session ID of the tunnel
session and sends it toward the Texit using normal tunnel
encapsulation.
The tunnel QUERY' message is processed hop by hop inside the tunnel
for the flow identified by the chosen tunnel flow ID. When Texit
receives the tunnel QUERY' message, it creates a reservation state
for the tunnel session without sending out a tunnel RESERVE' message
immediately.
The end-to-end QUERY message passes along tunnel intermediate nodes
just like any other tunneled packets. When Texit receives the end-
to-end QUERY message, it notices the binding of a tunnel session and
checks the state for the tunnel session. When the tunnel session
state is available, Texit updates the end-to-end QUERY message using
the tunnel session state, removes the tunnel BOUND_SESSION_ID object
and forwards the end-to-end QUERY message further along the path.
When Texit receives the first end-to-end RESERVE message issued by
the receiver, it finds the reservation state of the tunnel session
and triggers a tunnel RESERVE' message for that session. Meanwhile
the end-to-end RESERVE message will be appended with a tunnel
BOUND_SESSION_ID object and forwarded towards Tentry. When Tentry
receives the tunnel RESERVE', it creates the reservation state for
the tunnel session and MAY send a tunnel RESPONSE' back to Texit.
When Tentry receives the end-to-end RESERVE, it creates the end-to-
end reservation state and updates it with information from the
associated tunnel session reservation state. Then Tentry further
forwards the end-to-end RESERVE upstream toward the sender.
4.1.3. Sender-initiated Reservation for End-to-end and Receiver- initiated Reservation for Tunnel Signaling
Sender Tentry Tnode Texit Receiver
| | | | |
| RESERVE | | | |
+--------->| | | |
| | QUERY' | | |
| +=========>| | |
| | | QUERY' | |
| | +=========>| |
| | RESERVE | |
| +-------------------->| |
| | | RESERVE' | |
| | |<=========+ |
| | RESERVE' | | |
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| |<=========+ | |
| | RESPONSE'| | |
| +=========>| | |
| | | RESPONSE'| |
| | +=========>| |
| | | | RESERVE |
| | | +--------->|
| | | | RESPONSE |
| | | |<---------+
| | RESPONSE | |
| |<--------------------+ |
| RESPONSE | | | |
|<---------+ | | |
| | | | |
| | | | |
Figure 3: Sender-initiated Reservation for End-to-end and Receiver-
initiated Reservation for Tunnel Signaling
This scenario assumes the end-to-end signaling mode is sender-
initiated and the tunnel signaling mode is receiver-initiated.
Figure 3 shows the messaging flow of NSIS operation over IP tunnels
in this case. When Tentry receives the first end-to-end RESERVE
message from the sender, it chooses the tunnel flow ID, creates the
tunnel session and sends a tunnel QUERY' message matching the
requests of the end-to-end session toward the Texit. This Tunnel
QUERY' message SHOULD have the "RESERVE-INIT" bit set. Tentry also
appends to the original RESERVE message a tunnel BOUND_SESSION_ID
object containing the session ID of the tunnel session and sends it
toward Texit using normal tunnel encapsulation.
The tunnel QUERY' message is processed hop by hop inside the tunnel
for the flow identified by the chosen tunnel flow ID. When Texit
receives the tunnel QUERY' message, it creates a reservation state
for the tunnel session and immediately sends out a tunnel RESERVE'
message back to Tentry. When Tentry receives the tunnel RESERVE'
message it learns the outcome of the tunnel reservation and sends a
tunnel RESPONSE' message to Texit.
When Texit receives the end-to-end RESERVE message, it notices the
binding of a tunnel session and checks the state for the tunnel
session. It learns the outcome of tunnel session reservation from
the tunnel RESPONSE' message. Then it updates the end-to-end
reservation state using the tunnel session state, removes the tunnel
BOUND_SESSION_ID object and forwards the end-to-end RESERVE message
further along the path towards the receiver. When the end-to-end
reservation finishes, an end-to-end RESPONSE MAY be sent back from
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initiated and the tunnel signaling mode is sender-initiated.
Figure 4 shows the messaging flow of NSIS operation over IP tunnels
in this case. When Tentry receives the first end-to-end QUERY
message from the sender, it chooses the tunnel flow ID, creates the
tunnel session and sends a tunnel QUERY' message matching the request
of the end-to-end session toward the Texit. Tentry also appends to
the original QUERY message a tunnel BOUND_SESSION_ID object
containing the session ID of the tunnel session and sends it toward
the Texit using normal tunnel encapsulation.
The tunnel QUERY' message is processed hop by hop inside the tunnel
for the flow identified by the chosen tunnel flow ID. When Texit
receives the tunnel QUERY' message, it creates a reservation state
for the tunnel session without sending out a tunnel RESERVE' message
immediately.
The end-to-end QUERY message passes along tunnel intermediate nodes
just like any other tunneled packets. When Texit receives the end-
to-end QUERY message, it notices the binding of a tunnel session and
checks the state for the tunnel session. When the tunnel session
state is available, Texit updates the end-to-end QUERY message using
the tunnel session state, removes the tunnel BOUND_SESSION_ID object
and forwards the end-to-end QUERY message further along the path.
When Texit receives the first end-to-end RESERVE message issued by
the receiver, it finds the reservation state of the tunnel session.
Texit appends to the end-to-end RESERVE message a tunnel
BOUND_SESSION_ID object containing the matching tunnel session ID and
sends it upstream to Tentry.
When Tentry receives the end-to-end RESERVE message, it notices the
binding and immediately sends out a tunnel RESERVE' message matching
the end-to-end RESERVE request over the tunnel. This RESERVE'
message SHOULD include the Request Identification Information (RII)
to trigger a RESPONSE' from Texit.
When Tentry receives the result of tunnel reservation from the tunnel
RESPONSE' message, it updates the end-to-end RESERVE message and
forwards the end-to-end RESERVE message upstream to the Sender. The
Sender MAY send an end-to-end RESPONSE message to the receiver when
the whole process completes.
4.2. Implementation Specific Issues4.2.1. End-to-end and Tunnel Signaling Interaction
Given the two separate end-to-end and tunnel signaling sessions,
there are many ways of integrating the signaling of each session. In
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general, different interaction approaches can be grouped into
sequential mode and parallel mode. In sequential mode, end-to-end
signaling pauses when it is waiting for results of tunnel signaling,
and resumes upon receipt of the tunnel signaling outcome. In
parallel mode, end-to-end signaling continues outside the tunnel
while tunnel signaling is still in process and its outcome is
unknown. The operation outlined in Section 4.1 shows the sequential
mode. While this mode is suitable for a flow that requires hard
guarantee of tunnel reservation, it MAY not be the best choice for a
flow that can tolerate some QoS uncertainty but wants to complete the
signaling on the path as fast as possible. The parallel mode is
clearly for the latter case.
Having two separate signaling sessions also causes a possible race
condition. When an end-to-end session message carrying tunnel
binding object arrives at one of the tunnel endpoints, if the
corresponding tunnel session state has already been created, then the
tunnel endpoint can refer to information in the tunnel session state
(e.g., about tunnel reservation status, or tunnel resource
availability) and construct an end-to-end signaling message to be
sent out of the tunnel immediately. On the other hand, if the tunnel
endpoint receives an end-to-end signaling message carrying tunnel
binding referring to a tunnel session that does not yet exist, it MAY
either wait until the tunnel session information is ready, or forward
the end-to-end session signaling without waiting for the tunnel
session. If the end-to-end signaling indeed proceeds in the absence
of the tunnel session, the tunnel session MAY still be established
after some delay. Since the tunnel signaling message does not
contain its associated end-to-end session's session ID, it cannot
immediately change the state of its associated end-to-end session.
However, the next refresh of the corresponding end-to-end session
will carry the tunnel binding information and thus will update the
association of the end-to-end and the tunnel session state. If the
period waiting for the end-to-end signaling refresh is considered too
long, the tunnel endpoint MAY choose to actively poll the session
state table about the existence of tunnel session before the refresh
timer expires. In any case, once the end-to-end signaling session
learns about the tunnel signaling it can send an immediate refresh
out of the tunnel with knowledge of tunnel session.
The decision on whether and how long to wait for the corresponding
tunnel session information is implementation specific and controlled
by the tunnel endpoints. This document only requires that if an
NSIS-tunnel aware endpoint decides to go forward with the end-to-end
signaling outside the tunnel with an uncertain tunnel session
condition, it SHOULD indicate this in the corresponding end-to-end
signaling messages. As far as QoS NSLP is concerned, this means the
NON-QoSM Hop field [12] SHOULD be set to one. Note that in some
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cases, the application using NSIS signaling MAY wish to indicate the
preferred way of end-to-end and tunnel signaling interaction. For
example, an application that can not tolerate any QoS uncertainty
will prefer the sequential mode of operation; an application that has
a looser QoS requirement MAY prefer the parallel mode of operation
for faster signaling speed. Current NSIS specification does not
contain fields to convey this preference. New objects or flags will
need to be defined if this behavior is considered necessary.
4.2.2. Aggregate vs. Individual Tunnel Session Setup
The operation outlined in Section 4.1 applies to a flow that
qualifies an individual dynamic tunnel session. For a tunnel that
MAY contain multiple end-to-end sessions, it is more efficient to
keep aggregate tunnel sessions rather than individual tunnel sessions
whenever possible. This will save the cost of setting up a new
session and avoid the setup latency as well as the session
establishment race conditions mentioned above. Therefore, when the
tunnel endpoint creates a reservation for a tunnel session based on
the individual end-to-end session, it is up to local policy whether
it wants to actually create an aggregate session by requesting more
resources than the current end-to-end session requires. If it does,
other end-to-end sessions arrived later MAY make use of this
aggregate tunnel session. The tunnel endpoint will also need to
determine how long to keep the tunnel session if no active end-to-end
session is currently mapped to the aggregate tunnel session. The
decision MAY be based on knowledge of likelihood of traffic in the
future. It SHOULD be noted that once these kinds of on-demand
aggregate tunnel sessions are set up, they are treated the same as
pre-configured tunnel sessions to future end-to-end sessions.
Therefore, the adjustment of such aggregate sessions SHOULD follow
Section 5.
Note that the session ID of an aggregate tunnel session SHOULD be
different from that of the end-to-end session because they usually
have separate lifetime. If the tunnel endpoint is certain that the
tunnel session is for an individual end-to-end session alone, it MAY
in some cases want to reuse the same session ID for both sessions.
This will require additional manipulation of the NSLP state at the
tunnel endpoints, since the NSLP state is usually keyed based on the
session ID.
5. Protocol Operation with Pre-configured Tunnel Sessions
This section discusses NSIS operation over tunnels that are pre-
configured through management interface with one or more tunnel
sessions. A pre-configured tunnel sessions MAY be mapped to one
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session as an individual tunnel session but are usually mapped to
multiple end-to-end sessions as an aggregate tunnel session.
5.1. Tunnel with Exactly One Pre-configured Aggregate Session
If only one aggregate session is configured in the tunnel and all
traffic will receive the reserved tunnel resources, all packets just
need to be IP-in-IP encapsulated as usual. If there is only one
aggregate session configured in the tunnel but only some traffic
SHOULD receive the reserved tunnel resources through the aggregate
tunnel session, then the aggregate tunnel session SHOULD be assigned
an appropriate flow ID. Qualified packets need to be encapsulated
with this special flow ID. The rest of the traffic will be IP-in-IP
encapsulated as usual.
5.2. Tunnel with Multiple Pre-configured Aggregate Sessions
If there are multiple pre-configured aggregate sessions over a tunnel
set up, these sessions MUST be distinguished by their different
aggregate tunnel flow IDs. In this case it is necessary to
explicitly bind the end-to-end sessions with specific tunnel
sessions. This binding is conveyed between tunnel endpoints by the
tunnel BOUND_SESSION_ID object. Once the binding has been
established, Tentry SHOULD encapsulate qualified data packets
according to the associated aggregate tunnel flow ID. Intermediate
nodes in the tunnel will then be able to filter these packets to
receive reserved tunnel resources.
5.3. Adjustment of Pre-configured Tunnel Sessions
Adjustment of pre-configured tunnel sessions upon the change of its
mapped end-to-end sessions is related is up to local policy
mechanisms. RSVP-TUNNEL [16] described multiple choices to
accomplish this. First, the tunnel reservation is never adjusted,
which makes the tunnel a rough equivalent of a fixed-capacity
hardware link ("hard pipe"). Second, the tunnel reservation is
adjusted whenever a new end-to-end reservation arrives or an old one
is torn down ("soft pipe"). Doing this will require the Texit to
keep track of the resources allocated to the tunnel and the resources
actually in use by end-to-end reservations separately. The third
approach adopts some hysteresis in the adjustment of the tunnel
reservation parameters. The tunnel reservation is adjusted upwards
or downwards occasionally, whenever the end-to-end reservation level
has changed enough to warrant the adjustment. This trades off extra
resource usage in the tunnel for reduced control traffic and
overhead.
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The following lists basic tunnel related message processing rules for
selected end-to-end QoS NSLP messages working in the sequential
interaction mode. They are provided as references for implementors
to insure minimal interoperability.
6.1. End-to-end QUERY Message at Tentry
When an end-to-end QUERY message is received at Tentry, Tentry checks
whether the end-to-end session is entitled to tunnel resources.
If the end-to-end session SHOULD be bound to a tunnel session yet to
be created. Tentry creates a tunnel QUERY' message and sends it to
Texit. Tentry also appends a tunnel BOUND_SESSION_ID object to the
end-to-end QUERY message. The tunnel BOUND_SESSION_ID object
contains the session ID of the tunnel session. The end-to-end QUERY
message is then encapsulated and sent out through the tunnel
interface.
If the end-to-end session SHOULD be bound to an existing tunnel
session (whether aggregate or individual), Tentry appends a tunnel
BOUND_SESSION_ID object to the end-to-end tunnel QUERY message and
sends it toward Texit through the tunnel interface.
6.2. End-to-end QUERY Message at Texit
When an end-to-end QUERY message containing a tunnel BOUND_SESSION_ID
object is received, Texit creates a conditional reservation state for
the end-to-end session (i.e., a state is created but the related
outgoing signaling message, in this case the QUERY message, is held
until further information is available). It also checks to see if a
conditional reservation state for the associated tunnel session is
available. If yes, it reads information from the tunnel session
state and sends the end-to-end QUERY downstream. If the conditional
reservation state for tunnel session is not yet available, it will be
created upon receiving the tunnel QUERY', and then Texit SHOULD
forward the end-to-end QUERY downstream with information from results
of the tunnel QUERY'.
6.3. End-to-end RESERVE Message at Tentry6.3.1. Sender-initiated RESERVE Message
If the RESERVE message is received with its T-bit set (RESERVE tear),
Tentry removes the local state, then encapsulates the RESERVE message
and tunnels it to Texit. If there is a tunnel session associated
with this end-to-end session, Tentry also sends a tunnel RESERVE with
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T-bit set for that tunnel session.
If the end-to-end RESERVE message is a refresh for an existing end-
to-end session and this session is associated with a tunnel session,
the RESERVE message refreshes both two sessions. If the RESERVE
message causes changes in resources reserved for the end-to-end
session, depending on whether the tunnel signaling is sender
initiated or receiver initiated, Tentry SHOULD create a new tunnel
RESERVE' message or tunnel QUERY' message to start changing the
tunnel reservation as well. At the same time, Tentry appends a
tunnel BOUND_SESSION_ID object to the end-to-end RESERVE message and
sends it to Texit through the tunnel interface.
If the message is the first RESERVE message for an end-to-end
session, Tentry determines whether the end-to-end session is entitled
to tunnel resources based on policy control mechanisms outside the
scope of this document. If not, no special tunnel related processing
is needed. Otherwise, if this session SHOULD be bound to an existing
tunnel session (whether aggregate or individual), Tentry creates the
association between the end-to-end session and the tunnel session.
Then it appends a tunnel BOUND_SESSION_ID object to the end-to-end
RESERVE message and sends it through the tunnel interface (i.e. the
message is encapsulated and tunneled to Texit as normal).
If the end-to-end session SHOULD be bound to a tunnel session yet to
be created, Tentry assigns the tunnel flow ID, and constructs a
tunnel RESERVE' or QUERY' message, depending on whether the tunnel
signaling is sender initiated or receiver initiated. The QSPEC in
this tunnel message MAY be different from the original QSPEC, taking
into consideration the tunnel overhead of the encapsulation of data
packets. Tentry then associates the tunnel session with the end-to-
end session in the NSLP state and sends the tunnel message toward
Texit to start reserving resources over the tunnel. At the same
time, Tentry appends a tunnel BOUND_SESSION_ID object to the end-to-
end RESERVE message and sends it through the tunnel interface.
6.3.2. Receiver-initiated RESERVE Message
If the RESERVE message is received with its T-bit set (RESERVE tear),
Tentry removes the local state and forwards the message upstream. If
the tunnel signaling is sender initiated, Tentry also sends a tunnel
RESERVE' message to tear down the tunnel session.
If the end-to-end RESERVE message contains a tunnel BOUND_SESSION_ID
and is the first end-to-end RESERVE message, Tentry checks whether
the tunnel session bound to the end-to-end session indicated by the
RESERVE message already exists. If yes, Tentry records the
association between the end-to-end and the tunnel session, reads
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information from the tunnel session to create the end-to-end RESERVE
message to be forwarded upstream. If the state for the tunnel
session is not available yet, Tentry SHOULD create state information
for the tunnel session and indicate that a conditional reservation is
pending. If tunnel signaling is sender initiated, Tentry also sends
a tunnel RESERVE' message toward Texit to reserve tunnel resources.
When the actual tunnel session status is known at Tentry (from a
tunnel RESERVE' if tunnel signaling is receiver initiated or at
tunnel RESPONSE' if tunnel signaling is sender initiated) and if at
this time there is a pending reservation, Tentry SHOULD generate an
end-to-end RESERVE message and forward it upstream.
If the end-to-end RESERVE message contains a tunnel BOUND_SESSION_ID
and is a refresh, Texit refreshes the end-to-end session. If the
RESERVE message causes changes in resources reserved for the end-to-
end session and if tunnel signaling is sender initiated, Tentry sends
a tunnel RESERVE' message to Texit to change the reservation. In any
case, Texit checks the state information of the tunnel session. If
it finds that the reservation has been updated inside the tunnel,
Texit forwards the changed RESERVE message toward the sender. If the
tunnel reservation update failed, Texit MUST send a RESPONSE with
appropriate Error_Spec to the originator of the end-to-end RESERVE
message.
6.4. End-to-end RESERVE Message at Texit6.4.1. Sender-initiated RESERVE Message
If the end-to-end RESERVE message is received with its T-bit set
(RESERVE tear), Texit removes the local state, then forwards the
RESERVE message downstream. If tunnel signaling is receiver-
initiated, Texit also sends a tunnel RESERVE tear upstream toward
Tentry to tear down the tunnel session.
If the end-to-end RESERVE message contains a tunnel BOUND_SESSION_ID
and is the first end-to-end RESERVE message, Texit checks whether the
state for the tunnel session indicated by the RESERVE message already
exists. If yes, Texit records the association between the end-to-end
and the tunnel session and reads information from the tunnel session
to create the end-to-end RESERVE message to be forwarded downstream.
If the state for the tunnel session is not available yet, Texit
SHOULD create state information for the tunnel session and indicate
that a conditional reservation is pending. When the actual tunnel
RESERVE' or RESPONSE' message arrives, the tunnel session state will
be updated. If at this time there is a pending reservation, Texit
will generate an end-to-end RESERVE message and forwards it
downstream.
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If the end-to-end RESERVE message contains a tunnel BOUND_SESSION_ID
and is a refresh, Texit refreshes the end-to-end session. If the
RESERVE message causes changes in resources reserved for the end-to-
end session, Texit checks the state information of the tunnel
session. If the reservation has been updated inside the tunnel,
Texit forwards the RESERVE message toward the receiver. If the
tunnel reservation update failed, Texit MUST send a RESPONSE with
appropriate Error_Spec to the originator of the end-to-end RESERVE
message.
Note that the processing rules for end-to-end RESERVE at Texit in
end-to-end sender-initiated case is similar to those for end-to-end
RESERVE at Tentry in end-to-end receiver-initiated case.
6.4.2. Receiver-initiated RESERVE Message
If the RESERVE message is received with its T-bit set (RESERVE tear),
Texit removes the local state, then forwards the RESERVE message
upstream. If there is an individual tunnel session associated with
this end-to-end session, Texit also sends a tunnel RESERVE' with
T-bit set for that tunnel session.
Otherwise Texit checks to see if the end-to-end session is associated
with a tunnel session. If only conditional reservation state is
found and no actual reservation has been made, this RESERVE is the
first end-to-end RESERVE message. Texit appends a tunnel
BOUND_SESSION_ID object to this end-to-end RESERVE message and sends
it toward Tentry through the tunnel interface. Meanwhile if tunnel
signaling is receiver initiated Texit sends tunnel RESERVE' message
toward Tentry to reserve tunnel resources.
If the end-to-end session is bound to a tunnel session and the
RESERVE message is a refresh, it refreshes both the end-to-end
session and tunnel session. If the RESERVE message causes changes in
resources reserved for the end-to-end session and if tunnel signaling
is receiver initiated, Texit MAY create a new tunnel RESERVE' message
to change the tunnel reservation as well. Meanwhile, the end-to-end
RESERVE is appended with the tunnel BOUND_SESSION_ID object and sent
to Tentry through the reverse path.
6.5. Special Processing Rules for Tunnels with Aggregate Sessions
In situations where the end-to-end session is bound to aggregate
tunnel sessions, the handling is similar to that of RSVP-TUNNEL [16].
If the associated tunnel session is a "hard pipe" session, arrival of
a new end-to-end reservation or adjustment of an existing end-to-end
session MAY cause the overall resources needed in the tunnel session
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The Value field currently contains a single 'T' bit, indicating the
basic NSIS-tunnel scheme defined in this document. It is also
possible to use multiple bits to define NSIS-tunnel capability in
finer granularity. We have adopted the simplest approach by using
only one bit. The remaining reserved bits can be used to signal
other node characteristics in the future.
The bits marked 'A' and 'B' define the desired behavior for objects
whose Type field is not recognized. If a node does not recognize the
NODE_CHAR object, the desired behavior is "Ignore". That is, the
object MUST be deleted and the rest of the message processed as
usual. This can be satisfied by setting 'AB' to '01' according to
GIST specification .
This NODE_CHAR object is included in a QUERY or RESERVE message by a
tunnel endpoint who wishes to learn about the other endpoint's tunnel
handling capability. The other endpoint that receives this object
will know that the sending endpoint is NSIS-tunnel capable, and place
the same object in a RESPONSE message to inform the sending endpoint
of its own tunnel handling capability. The procedures for using
NODE_CHAR object in the four dynamically created tunnel session
scenarios are further detailed below.
If both end-to-end and tunnel sessions are sender-initiated
(Section 4.1.1) and Tentry is NSIS-tunnel capable, the Tentry
includes an RII object and a NODE_CHAR object with T bit set in the
first end-to-end RESERVE message sent to Texit. When Texit receives
this RESERVE message, if it supports NSIS tunneling, it learns that
Tentry is NSIS-tunnel capable and includes the same object with T bit
set in the RESPONSE message sent back to Tentry. Otherwise, Texit
ignores the NODE_CHAR object. When Tentry receives the RESPONSE
message, it learns whether Texit is NSIS-tunnel capable by examining
the existence of the NODE_CHAR object and its T-bit. If both tunnel
endpoints are NSIS-tunnel capable, the rest of the procedures will
follow those defined in Section 4.1.1. Alternatively, Tentry MAY
send out tunnel RESERVE message before the RESPONSE message
confirming the NSIS-tunnel capability of Texit is received. If later
it learns that the Texit is not NSIS-tunnel capable, it SHOULD send
out teardown messages to cancel the tunnel session reservation that
has already been made. This way the signaling process is faster when
Texit is NSIS-tunnel capable, but it can lead to temporary waste of
tunnel resources if Texit is not NSIS-tunnel capable.
If both end-to-end and tunnel sessions are receiver-initiated
(Section 4.1.2) and Tentry is NSIS-tunnel capable, the Tentry
includes an RII object and a NODE_CHAR object with T bit set in the
first end-to-end QUERY message sent toward Texit. An NSIS-tunnel
capable Texit learns from the NODE_CHAR object whether Tentry is
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NSIS-tunnel capable. In reply to this end-to-end QUERY message, the
NSIS-tunnel capable Texit includes a NODE_CHAR object with T bit set
in its RESPONSE message to notify Tentry of its own tunnel
capability. If both tunnel endpoints are NSIS-tunnel capable, the
rest of the procedures will follow those defined in Section 4.1.2.
Otherwise, Texit will not initiate tunnel session reservations.
If the end-to-end session is sender-initiated, the tunnel session is
receiver-initiated (Section 4.1.3), and Tentry is NSIS-tunnel
capable, the Tentry includes an RII object and a NODE_CHAR object
with T bit set in the first end-to-end RESERVE message sent toward
Texit. An NSIS-tunnel capable Texit learns from the NODE_CHAR object
whether Tentry is NSIS-tunnel capable. In reply to this end-to-end
QUERY message, the NSIS-tunnel capable Texit includes a NODE_CHAR
object with T bit set in its RESPONSE message to notify Tentry of its
own tunnel capability. If both tunnel endpoints are NSIS-tunnel
capable, the rest of the procedures will follow those defined in
Section 4.1.3. Otherwise, Texit will not initiate tunnel session
reservations.
If the end-to-end session is receiver-initiated, the tunnel session
is sender-initiated (Section 4.1.4), and Tentry is NSIS-tunnel
capable, the operation is similar to the case where both sessions are
receiver-initiated. The Tentry includes an RII object and a
NODE_CHAR object with T bit set in the first end-to-end QUERY message
sent toward Texit. An NSIS-tunnel capable Texit learns from the
NODE_CHAR object whether Tentry is also NSIS-tunnel capable. In
reply to this end-to-end QUERY message, the NSIS-tunnel capable Texit
includes a NODE_CHAR object with T bit set in its RESPONSE message to
notify Tentry of its own tunnel capability. If both tunnel endpoints
are NSIS tunnel capable, the rest procedures follow those defined in
Section 4.1.4. Otherwise, Tentry will not initiate further NSIS
tunnel session reservations.
8. Other Considerations8.1. Other Types of NSLP
This document discusses tunnel operation using QoS NSLP. It will be
desirable to have the scheme work with other NSLPs as well. Since
NSIS-tunnel operation involves specific NSLP itself and different
NSLPs have different message exchange semantics, the NSIS-tunnel
specification would not be the same for all NSLPs. However the basic
aspects behind NSIS-tunnel operation could indeed be similar for
different types of NSLPs. For example, in the case of NATFW NSLP
[13], the most important signaling operation is CREATE. Assuming
Tentry is a NATFW NSLP, the tunnel handling for the CREATE operation
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is expected to be very similar to the sender-initiated QoS
reservation case. There are also a number of reverse directional
operations in NATFW NSLP, such as RESERVE_EXTERNAL_ADDRESS and
UCREATE. It is not very clear whether IP tunnel will cause problems
with these messages in general. But they are likely easier to deal
with than the receiver-initiated reservation case in QoS NSLP. This
topic will be discussed in future version of this document if
necessary.
8.2. IPSEC Flows
If the tunnel supports IPSEC (especially ESP in Tunnel-Mode with or
without AH), it MAY use the flow label, DSCP field, or IPSEC SPI
along with the tunnel source and destination address, as discussed in
Section 3.1 to form the tunnel Flow ID. All these are standard NSIS
MRI fields that can be matched by the NSIS packet classifier.
Virtual destination ports as in RSVP-IPSEC [17] MAY be defined for
further flow demultiplexing capability at the destination side if
necessary.
8.3. NSIS-tunnel Operation and Mobility
NSIS-tunnel operation needs to interact with IP mobility in an
efficient way. In places where pre-configured tunnel sessions are
available, the process is relatively straightforward. For dynamic
individual signaling tunnel sessions, one way to improve NSIS
mobility efficiency in the tunnel is to reuse the session ID of the
tunnel session when tunnel flow ID changes during mobility. This
works as follows. With a mobile IP tunnel, one tunnel endpoint is
the Home Agent (HA), and the other endpoint is the Mobile Node (MN)
if collocated Care-of-Address (CoA) is used, or the Foreign Agent
(FA) if FA CoA is used. When MN is a receiver, Tentry is the HA and
Texit is the MN or FA. In a mobility event, handoff tunnel signaling
messages will start from HA, which MAY use the same session ID for
the new tunnel session. When MN is a sender and collocated CoA is
used, Tentry is the MN and Texit is the HA. Handoff tunnel signaling
is started at the MN. It MAY also use the session ID of the previous
tunnel session for the new tunnel session. When MN is a sender and
FA CoA is used, the situation is complicated because Tentry has
changed from the old FA to the new FA. In this case the new FA does
not have the session ID of the previous tunnel session.
When mobile IP is operating on a bi-directional tunneling mode, NSIS-
tunnel operation with mobility MAY be further improved by localizing
the handoff tunnel signaling process by bypassing the path between HA
and CN.
General aspects of NSIS interaction with mobility are discussed in
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[14].
9. Security Considerations
This draft does not draw new security threats. Security
considerations for NSIS NTLP and QoS NSLP are discussed in [2] and
[3], respectively. General threats for NSIS can be found in [18].
10. Appendix10.1. Various Design Alternatives10.1.1. End-to-end and Tunnel Signaling Interaction Model
The contents of original end-to-end singling messages are not
directly examined by tunnel intermediate nodes. To carry out tunnel
signaling we choose to maintain a separate tunnel session for the
end-to-end session by generating tunnel specific signaling messages.
An alternative approach is to stack tunnel specific objects on top of
the original end-to-end messages and make these messages visible to
tunnel intermediate nodes. Thus, these new messages serve both the
end-to-end session and tunnel session. This approach turns out to be
difficult because the actual tunnel signaling messages differ from
the end-to-end signaling message both in GIST layer and NSLP layer
information, such as MRI, PACKET CLASSIFIER and QSPEC. Although
QSPEC can be stacked in an NSLP message, there doesn't seem to be a
handy way to stack MRI and the PACKET CLASSIFIER in the NSLP layer.
In addition, the stacking method only applies to individual signaling
tunnels.
The separate end-to-end tunnel session signaling model adopted in
this document handles both individual and aggregate signaling tunnels
in a consistent way. Its major drawback is the race condition we
mentioned in Section 4.2. However, we defined simple rules to solve
this problem while maintaining interoperability.
This document defines the sequential and parallel modes of end-to-end
and tunnel signaling interaction. There are a number of different
aspects that can result in variations in carrying out the actual
interaction. One aspect is the tunnel session initiation location.
For example, it is possible to initiate the tunnel session from
Texit, instead of Tentry as in the proposed scheme. A second aspect
is the tunnel session initiation time point. For example, in cases
when both end-to-end session and tunnel session are receiver-
initiated, it is possible to start the tunnel session when Tentry
receives the first end-to-end RESERVE message, instead of when Tentry
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receives the first end-to-end QUERY message, as in the proposed
scheme. The advantage of our adopted approach is that it will allow
the first end-to-end QUERY message to also gather tunnel
characteristics along with the rest of the end-to-end path. A third
aspect is how the tunnel signaling messages are used. For example,
in the case where end-to-end session is receiver initiated and tunnel
session is sender initiated (Section 4.1.4), the first tunnel QUERY'
message sent after receiving the end-to-end QUERY message by Tentry
can be replaced by a tunnel RESERVE' message, if the application
wants to trade temporary oversized or wasted (if the end-to-end
reservation turns out to be unsatisfied) tunnel resource reservations
for faster signaling setup delay. All these aspects are local
optimization issues. We require any implementation to support the
basic scheme defined in the main text of document to allow
interoperability.
10.1.2. Packet Classification over the Tunnel
Packet classification over the tunnel MAY be done in either of the
two ways: first, retaining the end-to-end packet classification
rules; Second, using tunnel specific classification rules. In the
first approach, tunnel packet classification is not tied with tunnel
MRI. This is a useful property especially in handling tunnel
mobility. Mobility changes the tunnel MRI, if at the same time the
packet classification rule does not change, the common path after a
handoff does not need to be updated about the packet classification,
which results in a better handoff performance. The main problem with
this approach is that most existing routers do not support inspection
of inner IP headers in an IP tunnel, where the tunnel independent
packet classification fields usually reside. Therefore this document
adopts the second approach which does not pose special classification
requirements on intermediate tunnel nodes.
10.1.3. Tunnel Binding Methods
In this document, the end-to-end session and its mapping tunnel
session use different session IDs and they are associated with each
other using the BOUND_SESSION_ID object. This choice is obvious for
aggregate tunnels sessions because in that case the original end-to-
end session and the corresponding aggregate tunnel session require
independent control.
Sessions in individual signaling tunnels are created and deleted
along with the related end-to-end session. So association between
the end-to-end session and the corresponding individual tunnel
session has another choice: the two sessions MAY share the same
session ID. Instead of sending a BOUND_SESSION_ID object, it MAY be
possible to define a BOUND_FLOW_ID object, to bind the flow ID of the
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end-to-end session to the flow ID of the tunnel session at the tunnel
endpoints. However, since flow ID is usually derived from MRI, if a
NAT is present in the tunnel, this BOUND_FLOW_ID object will have to
be modified in the middle, which makes the process fairly
complicated. Furthermore, it is not desirable to have different
session association mechanisms for aggregate signaling tunnels and
individual signaling tunnels. Therefore, we decide to use the same
tunnel BOUND_SESSION_ID mechanism for both individual and aggregation
tunnel sessions. Note that in this case the mobility handling inside
the tunnel can still be optimized in certain situations as discussed
in Section 8.3.
In this document we used the existing BOUND_SESSION_ID object with a
tunnel Binding_code to indicate the reason of binding. Two other
options were considered.
1. Define a designated "tunnel object" to be included when the
tunnel binding needs to be conveyed.
2. Define a "tunnel bit" in corresponding NSLP message headers.
These options are not chosen because they either requires the
creation of an entirely new object, or the change of basic message
headers. They are also not generic solutions that can cover other
binding causes.
There are basically three ways to carry the binding object between
Tentry and Texit, using (a) end-to-end signaling messages, (b) tunnel
signaling messages, (c) both end-to-end and tunnel signaling
messages. In option (a) only tunnel endpoints see the tunnel binding
information. In option (b), every tunnel intermediate node sees the
binding information. Since there will be no state for the end-to-end
session in tunnel intermediate nodes, they will all generate a
message containing an "INFO_SPEC" object indicating no bound session
found according to [3], which is not desirable. Option (c) has an
advantage that if both end-to-end and tunnel signaling messages have
tunnel binding information, the racing condition will be resolved
faster. However it suffers the same problem as in (b). Therefore
the choice in this document for carrying the tunnel binding object is
option (a).
11. Acknowledgements12. References12.1. Normative ReferencesShen, et al. Expires April 24, 2007 [Page 29]

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